Chemical and photochemical degradation of human hair: a free-volume microprobe study

The microstructural changes in human hair due to chemical and photochemical oxidative processes have been monitored in terms of free volume employing Positron Annihilation Lifetime Spectroscopy (PALS). The results show that upon UV exposure the photosensitive amino acid residues present in the amorphous domains of virgin/bleached hair cross-link under ambient conditions. Further, the bleached hair readily undergoes photodamage in comparison with the virgin hair implying the diminished photoprotective action of the melanin granules within it. Swelling of hair fiber was evident in the early stages of UV exposure (<300h), bleaching, and humidification subsequent to irradiation. Swelling and cross-linking were the two main processes observed following UV exposure, which oppositely affect the free volume holes size. Supplementary techniques such as DSC and XRD were used to support/extend the interpretation of the PALS results. The UV irradiation resulted in reduction of the average crystallite size in hair, which is attributed to the possible fragmentation of protein domains. The present work is the first positron lifetime measurement on human hair that demonstrates the ability of PALS to provide information on hair damage at molecular level, a vital input for cosmetic industry and applicable to biopolymers research as well.